In recent years, lithium-ion rechargeable batteries have been increasingly required for intended uses such as portable information terminal, portable electronic equipment, electric cars, hybrid electric cars, and further, a stationary power storage system. However, conventional lithium-ion rechargeable batteries comprise a flammable organic solvent as a liquid electrolyte, and thus, the conventional lithium-ion rechargeable batteries need strong exterior materials for preventing the leakage of the organic solvent. In addition, in the case of portable personal computers and the like, these devices need to have a structure for coping with a possible risk of the leakage of such a liquid electrolyte. Hence, the lithium-ion rechargeable batteries would cause restriction to the structures of devices.
Moreover, the intended use of the lithium-ion rechargeable batteries has widened to moving bodies such as automobiles or airplanes, and stationary-type lithium-ion rechargeable batteries have been required to have a high volume. Under such circumstances, safety has been more emphasized than before, and it has been focused on the development of all-solid-state lithium-ion rechargeable batteries, which do not comprise harmful substances such as organic solvents.
As a solid electrolyte used in such all-solid-state lithium-ion rechargeable batteries, the use of an oxide, a phosphorus compound, an organic polymer, a sulfide, etc. has been studied.
However, an oxide or a phosphorus compound has such properties that the particles thereof are hard. Accordingly, when a solid electrolyte layer is molded using such a material, it is generally necessary to sinter it at a high temperature of 600° C. or higher, and thus, it takes labor and time. Furthermore, when an oxide or a phosphorus compound is used as a material for a solid electrolyte layer, it is disadvantageous in that the interfacial resistance between the material and an active material is increased. The organic polymer is disadvantageous in that it has low lithium ion conductivity at room temperature, and as the temperature is decreased, the conductivity is drastically decreased.
Regarding a novel Li-ion solid state conductor, it has been reported in 2007 that the high-temperature phase of LiBH4 has high lithium ion conductivity (Non Patent Literature 1). Since LiBH4 has a low density, a light battery can be produced when such LiBH4 is used as a solid electrolyte. Further, since LiBH4 is stable even at a high temperature (e.g., approximately 200° C.), it is also possible to produce a heat-resistant battery using the LiBH4.
However, LiBH4 is problematic in that its lithium ion conductivity is largely decreased at lower than the phase transition temperature, 115° C. As such, in order to obtain a solid electrolyte having high lithium ion conductivity even at lower than the phase transition temperature, 115° C., a solid electrolyte prepared by combining LiBH4 with an alkaline metal compound has been proposed. For example, in 2009, it has been reported that a solid solution prepared by adding LiI to LiBH4 is able to keep a high-temperature phase even at room temperature (Non Patent Literature 2 and Patent Literature 1).
As a further means for improving lithium ion conductivity, it has been proposed to use, as a solid electrolyte, a glass obtained by mixing a sulfide solid electrolyte 0.75 Li2S-0.25 P2S5 with LiBH4 and subjecting the mixture to a mechanical milling treatment (Non Patent Literature 3). This glass solid electrolyte has high lithium ion conductivity at room temperature (1.6×10−3 S/cm), but since it mainly comprises a sulfide solid electrolyte, it has a high density, and thus, the glass solid electrolyte is disadvantageous that a solid electrolyte layer has a high weight, when the glass solid electrolyte is used to form the solid electrolyte layer. Further, the glass solid electrolyte is also problematic in that the interfacial resistance between a current collector or an electrode layer (hereinafter, a positive electrode layer and a negative electrode layer are collectively referred to as an “electrode layer” at times) and a solid electrolyte layer is increased, when such a solid electrolyte layer is used to produce an all-solid-state battery.